Leptin, produced by white adipose tissue, is critical in the regulation of energy balance. Leptin decreases foot intake while current increasing thermogenesis and energy expenditure. Starvation, increases food intake, inhibits energy expenditure, and rapidly lowers leptin levels, the behavioral, neuroendocrine, and autonomic effects of leptin (or lack of it) are likely mediated by pathways originating in the arcuate nucleus of the hypothalamus (Arc). The Arc contains neurons that express leptin receptors, neuropeptide Y (NPY), agouti related peptide (AgRP), pro- opiomelanocortin (POMC), and cocaine- and amphetamine-regulated transcript (CART). All 4 of these neuropeptides are regulated by starvation and leptin. However, the efferent projections of starvation and leptin regulated neuropeptides in the Arc are not known. Melanin concentrating hormone (MCH) and orexin (ORX) neurons diffusely innervate the neuraxis including monosynaptic projections to the cerebral cortex and to autonomic pre-ganglionic neurons in the medulla and spinal cord. The mRNAs for both MCH and ORX rise following fasting and MCH and ORX peptides increase feeding behavior when administered centrally. Thus, MCH and ORX neurons are ideally positioned to regulated cognitive and autonomic aspects of food intake and body weight. MCH and ORX neurons receive specific innervation from AgRP, POMC, and NPY neurons, thus linking medial and lateral hypothalamic feeding centers. In this proposal, we outline experiments designed to characterize the neuroanatomic mechanisms by which leptin can blunt starvation-induced changes in key neuropeptide systems originating in the Arc that regulated food intake and energy expenditure. First, we will determine the CNS sites innervated by neurons containing starvation-regulated peptides in the Arc. This will be accomplished using retrograde racing and in situ hybridization for CART, POMC, NPY, and AgRP. We will quantify the mRNA levels of these 4 neuropeptides in retrogradely labeled neurons following fasting with and without leptin replacement. Second, using retrograde tracing, and in situ hybridization for immediately early genes, we will determine if subpopulations of starvation- and leptin-regulated neurons in the Arc project to the lateral hypothalamic area and to autonomic pre-ganglionic neurons in the spinal cord and medulla. Third, using anterograde tracer injections into the Arc, retrograde tracer injections into the cerebral cortex, medulla and spinal cord, we will determine the CNS sites innervated by starvation-regulated MCH and ORX neurons. We will also determine if ORX and MCH neurons innervating key autonomic regulatory sites in the cerebral cortex, brainstem, and spinal cord also receive innervation from Arc neurons. Fourth, using electron microscopy and immunocytochemistry we will determine if neurons in the Arc make synaptic contacts with ORX and MCH cells in the LHA and with autonomic pre-ganglionic neurons in the spinal cord and medulla.